Interstate 40/State Route 66 Diverging Diamond Interchange

A Gem of a Junction

DDI intersections are shaped like a steep “X” and drivers are pointed into the opposite lanes. When a motorist pulls up to this intersection, the signal is either green for southbound traffic to move, or green for northbound movement. By only having two signaling phases at each intersection, more traffic can flow through the intersection during each cycle. By eliminating left turns, dangerous T-bone crashes don’t occur, creating a much safer intersection.

In recent years, the interchange between Interstate 40 and state Route 66 at Exit 407 in Sevier County has more than exceeded its capacity to keep traffic moving smoothly due to the increasing volume of tourists passing through the region. The interchange is the gateway to the Great Smoky Mountains National Park—the most visited national park in the country with approximately 10 million visitors a year. Needing to increase capacity and safety at the heavily traveled exchange, Tennessee Department of Transportation (TDOT) called upon longtime trusted advisor GS&P.

“The existing interchange had been performing poorly for many years, and traffic flow through Pigeon Forge, Gatlinburg and the Smoky Mountains was only getting worse,” explains senior civil engineer Jonathan Haycraft. “TDOT’s overarching goal was to address the high volumes of traffic and crash rates, as well as the projected ramp queues within the vicinity of the exchange, while keeping disruptions to tourist traffic at a minimum.”

“Another motivating factor in redesigning this interchange was the ongoing construction TDOT was undertaking to widen SR 66 and add more lanes to alleviate traffic,” adds senior transportation engineer Jason Brady. “Improving the existing interchange would ultimately complement the other roadwork and better manage the volume of motorists traveling to and from vacation.”

Due to the urgent need for improvements, TDOT requested the project be completed on an extremely fast-track schedule.

“GS&P was given six months to complete the effort,” says Haycraft. “In comparison, a project of this magnitude would typically take a standard roadway design team of two to six people anywhere from 18 to 24 months to complete. To meet the accelerated timetable, we assembled a massive design team consisting of 20 staff members from three different GS&P office locations.”

The Diverging Diamond Solution

The existing exchange at Exit 407 was a standard diamond interchange—a common type of junction where one highway crosses over another. Two separate bridges—one for each direction of travel—conveyed SR 66’s northbound and southbound lanes over I-40. The highest volume of turning traffic was movement from northbound SR 66 to westbound I-40. To reduce construction time, GS&P determined it would be highly beneficial to keep the existing bridges in place.

After exploring multiple design possibilities, it quickly became evident that a Diverging Diamond Interchange (DDI) was the clear solution for the location due to its increased capacity to handle traffic, cost effectiveness, reduced construction time, and minimized disruption to tourism.

“TDOT’s planning study revealed that a DDI would provide the most efficient traffic pattern for people leaving the tourist areas and heading back in the direction of Knoxville or Nashville,” explains Haycraft. “With a standard diamond interchange, you have two or three lanes in each direction and two signals with multi-lane left turns. By using a DDI, we were able to keep the signals timed to just stop-and-go cycles with no turn movements, which moves a lot more vehicles in a shorter amount of time.”

Improved safety was also a key factor in selecting the Diverging Diamond Interchange. Brady explains:

“The easiest way to describe a DDI is that when you drive it, it feels more like two one-way streets that intersect rather than an interchange. When properly designed with the right geometrics, if you just follow the stop-and-go signals, you’re going to be pointed in the direction you’re meant to travel in. And you have the ability to make a free-flow left onto the interstate—you’re never turning across traffic, which was the case with the previous exchange. Eliminating the turn arrows not only helps efficiency, but also greatly increases safety by reducing the possibility of T-bone collisions, which is the most dangerous type of car crash.”

To further increase capacity, GS&P redesigned the markings on the existing two-lane bridges to reduce the width of lanes and shoulders, and to add a third lane to each bridge. Repurposing the current bridges not only saved on costs, but also helped reduce the project’s impact on the environment and local community.

“Utilizing the existing bridges reduced the amount of construction materials hauled on and off the project site as well as the resulting truck emissions,” says Haycraft. “The improved interchange further reduces emissions by leaving fewer vehicles idling at stop lights, and the increased traffic flow means more visitors to the national park as well as to local businesses.”

“A DDI adds a level of efficiency to traffic movement,” notes Brady. “The only thing comparable was looking at a three-level diamond interchange, but the price tag would have been much higher for a similar result and level of service.”

Though GS&P’s design decisions saved money and helped expedite construction, the team didn’t cut any corners on quality.

“Despite the abbreviated schedule, we took extra time even in the crunch to analyze traffic and lane flow to see how we could improve upon the concept,” says transportation engineer Cody Crews. “We could have just executed the basic job we were asked to perform, but it’s a part of our culture to go above and beyond and deliver a better product—even when our entire team is under the gun.”

Public Outreach and Information

GS&P developed a comprehensive public information program to demonstrate to local motorists and tourists alike how the new interchange would work, primarily by displaying video simulations in person at local events and on television newscasts.

“Videos and images are effective in conveying how the interchange operates, but the best way to understand a DDI is simply to drive one,” says Haycraft. “Once people have a chance to experience it for themselves, they realize just how well it works.

“In our industry, lack of feedback is the best kind of feedback. And we’ve gone through an entire tourist season without a single complaint about the new interchange.”

“The improvements to this exchange are most notable during tourist season,” adds Brady. “Where drivers used to wait 30 minutes for 10 to 20 signal cycles, they can now make it through the interchange in just one cycle.”

One Team, One Unit

Opened to traffic in June 2015, the Diverging Diamond Interchange between Interstate 40 and state Route 66 at Exit 407 provides a safer and more efficient interchange for travelers to and from Sevier County. GS&P completed the design and construction plan in just under six months. Construction for the project was completed two weeks ahead of schedule.

“Because of the time constraints, we started parts of the process at the beginning of the project that we typically wouldn’t have begun until the design was 60 to 90 percent complete,” says Crews. “Our team in Knoxville was working on traffic control at the same time we were still working on what we were going to build. The biggest challenge—and ultimately the biggest victory—was making sure everyone stayed on the same page. Everybody worked together on simultaneous parts of the project as one team and one unit. We truly lived the firm’s core values of commitment and teamwork to make the project a success.”

“It’s not very often a client like TDOT comes to you and says, ‘We need you to design a very complex and unique project in a third of the time it would typically take, and develop the plans so the contractor can build it in half the normal time. And, by the way, it’s the interchange that feeds tourists to the most heavily visited national park in the U.S.,’” says Haycraft. “That request shows that our client trusts in our management skills and our capabilities to be on the leading edge of innovation. We not only delivered for our client—we beat their expectations.”

Gresham, Smith and Partners is a privately held firm of architects, engineers, interior designers and planners creating design and consulting solutions for the built environment. Our more than 800 employees are committed to building value with our clients as trusted advisors and to making positive social, economic and environmental impacts in our communities. In 2017, GS&P proudly celebrates 50 years in business. We look forward to continuing to serve our clients while fostering the professional and personal growth of our employees.